Koch, a Democrat, represented the 17th congressional district of New York in the U.S. House of Representatives for two terms (1969-1972) and the 18th CD for two and a half terms (1973-1977).

In 1977, Koch beat incumbent New York City mayor Abraham Beame in the Democratic primary election and then went on to win the general election in November. He was re-elected in 1981 and 1985 but was defeated in his bid for a fourth term in 1989, losing the Democratic primary to David Dinkins.